As a curing agent exhibiting low-temperature fast-curing activity for an epoxy resin, a micro-encapsulated aluminum chelate latent curing agent has previously been proposed which holds an aluminum chelating agent in a porous resin obtained by interfacial polymerization of a polyfunctional isocyanate compound (Patent Document 1).
With a thermosetting epoxy resin composition that contains an epoxy compound and an aluminum chelate latent curing agent like that disclosed in Patent Document 1 for curing the epoxy compound, silanolate anions derived from a silane coupling agent need to be present to activate the curing agent. However, due to the problem that the silanolate anions attach to the β-position carbon of the epoxy group in the epoxy compound, thereby causing a polymerization termination reaction to occur, it is difficult to perform polymerization without the glycidyl ether epoxy compound, which is susceptible to a β-carbon addition reaction, causing a polymerization termination reaction to occur. Consequently, there is the problem that an alicyclic epoxy compound, which is not susceptible to an addition reaction onto the β-position carbon by silanolate anions, has to be used despite the high cost.
Therefore, in order to enable low-temperature fast-curing of a glycidyl ether epoxy compound with an aluminum chelate latent curing agent like that disclosed in Patent Document 1, it has been proposed that the mechanical nature of a porous microcapsule is improved and the thermal response speed during curing of the epoxy compound is increased (Patent Document 2). This can be achieved by using in the thermosetting epoxy resin composition an aluminum chelate latent curing agent as well as a highly sterically hindered silanol compound that has a specific chemical structure and that has not been used conventionally as a silane coupling agent, and further, when performing the interfacial polymerization of the polyfunctional isocyanate compound, by allowing both a radical polymerizable monomer, such as divinyl benzene, and a radical polymerization initiator to be copresent and copolymerized.